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Abstract. We present the first results from simulations of processes leading to planet formation in protoplanetary disks with different metallicities. For a given metallicity, we construct a two-dimensional grid of disk models with different initial masses and radii (M 0 , R 0 ). For each disk, we follow the evolution of gas and solids from an early evolutionary stage, when all solids are in the form of small dust grains, to the stage when most solids have condensed into planetesimals. Then, based on the core accretion -gas capture scenario, we estimate the planet-bearing capability of the environment defined by the final planetesimal swarm and the still evolving gaseous component of the disk. We define the probability of planet-formation, P p , as the normalized fractional area in the (M 0 , log R 0 ) plane populated by disks that have formed planets inside 5 AU. With such a definition, and under the assumption that the population of planets discovered at R < 5 AU is not significantly contaminated by planets that have migrated from R > 5 AU, our results agree fairly well with the observed dependence between the probability that a star harbors a planet and the star's metal content. The agreement holds for the disk viscosity parameter α ranging from 10 −3 to 10 −2 , and it becomes much poorer when the redistribution of solids relative to the gas is not allowed for during the evolution of model disks.

Abstract.We have developed and applied a model designed to track simultaneously the evolution of gas and solids in protoplanetary disks from an early stage, when all solids are in the dust form, to the stage when most solids are in the form of a planetesimal swarm. The model is computationally efficient and allows for a global, comprehensive approach to the evolution of solid particles due to gas-solid coupling, coagulation, sedimentation, and evaporation/condensation. The co-evolution of gas and solids is calculated for 10 7 yr for several evolution regimes and starting from a comprehensive domain of initial conditions. The output of a single evolutionary run is a spatial distribution of mass locked in a planetesimal swarm. Because swarm's mass distribution is related to the architecture of a nascent planetary system, diversity of swarms is taken as a proxy for a diversity of planetary systems. We have found that disks with low values of specific angular momentum are bled out of solids and do not form planetary systems. Disks with high and intermediate values of specific angular momentum form diverse planetary systems. Solar-like planetary systems form from disks with initial masses ≤0.02 M and angular momenta ≤3 × 10 52 g cm 2 s −1 . Planets more massive than Jupiter can form at locations as close as 1 AU from the central star according to our model.

[1] The presence of valley networks (VN) on Mars suggests that early Mars was warmer and wetter than present. However, detailed geomorphic analyses of individual networks have not led to a consensus regarding their origin. An additional line of evidence can be provided by the global pattern of dissection on Mars, but the currently available global map of VN, compiled from Viking images, is incomplete and outdated. We created an updated map of VN by using a computer algorithm that parses topographic data and recognizes valleys by their morphologic signature. This computer-generated map was visually inspected and edited to produce the final updated map of VN. The new map shows an increase in total VN length by a factor of 2.3. A global map of dissection density, D, derived from the new VN map, shows that the most highly dissected region forms a belt located between the equator and mid-southern latitudes. The most prominent regions of high values of D are the northern Terra Cimmeria and the Margaritifer Terra where D reaches the value of 0.12 km À1 over extended areas. The average value of D is 0.062 km À1 , only 2.6 times lower than the terrestrial value of D as measured in the same fashion. These relatively high values of dissection density over extensive regions of the planet point toward precipitation-fed runoff erosion as the primary mechanism of valley formation. Assuming a warm and wet early Mars, peculiarity of the global pattern of dissection is interpreted in the terms of climate controlling factors influenced by the topographic dichotomy.

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Radial velocity observations of the F8 V star t Andromedae taken at Lick and at Whipple Observatories have revealed evidence of three periodicities in the line-of-sight velocity of the star. These periodicities have been interpreted as evidence for at least three low-mass companions (LMCs) revolving around t Andromedae. The mass and orbital parameters inferred for these companions raise questions about the dynamical stability of the system. We report here results from our independent analysis of the published radial velocity data, as well as new unpublished data taken at Lick Observatory. Our results conÐrm the Ðnding of three periods in the data. Our best Ðts to the data, on the assumption that these periods arise from the gravitational perturbations of companions in Keplerian orbits, are also generally in agreement but with some di †erences from the earlier Ðndings. We Ðnd that the available data do not constrain well the orbital eccentricity of the middle companion in a three-companion model of the data. We also Ðnd that in order for our best-Ðt model to the Lick data to be dynamically stable over the lifetime of the star (D2 billion years), the system must have a mean inclination to the plane of the sky greater than 13¡. The corresponding minimum inclination for the best Ðt to the Whipple data set is 19¡. These values imply that the maximum mass for the outer companion can be no greater than about 20 Jupiter masses. Our analysis of the stability of the putative systems also places constraints on the relative inclinations of the orbital planes of the companions. We comment on global versus local (i.e., method of steepest descent) means of Ðnding best-Ðt orbits from radial velocity data sets.